1. Field of the Invention
The present invention relates to a method of making high temperature, erosion resistant coatings used as abradable seals and thermal barrier coatings.
2. Background Information
Most components of combustion turbines are operated at very high temperatures and often require the use of specialized coatings/inserts to protect underlying supporting materials. These specialized coating/inserts include thermal barrier coatings (TBCs), turbine blade tip inserts, and abradable seals disposed opposite the turbine blade tips, as taught in U.S. Pat. No. 5,180,285 (Lau).
Conventional TBCs typically comprise a thin layer of zirconia. In many applications, the coatings must be erosion resistant and must also be abradable. For example, turbine ring seal segments, which fit with tight tolerances against the tips of turbine blades, must withstand erosion and must preferentially wear or abrade in order to reduce damage to the turbine blades, and form a tight seal with the turbine blade. Protective coating system can include several layers including a metallic bond or barrier coating of MCrAlY having an alumina scale and, for example, a columnar yttria stabilized zirconia thermal barrier, as taught in U.S. Pat. No. 4,916,022 (Solfest et al.), which can be further coated by an erosion resistant layer of alumina or silicon carbide, applied by physical vapor deposition techniques, as taught by U.S. Pat. No. 5,683,825 (Bruce et al.).
In U.S. Pat. No. 5,780,146 (Mason et al.), 30 wt. % to 50 wt. % (50 vol. % to 60 vol. %) of hollow alumino silicate or alumina spheres of 400 micrometer to 1800 micrometer diameter, and having a high temperature capability of approximately 1300° C., were used in an aluminum phosphate matrix, for an abradable seal. The seal is used over a ceramic matrix composite shroud segment, which may comprise silicon carbide fibers in an alumina matrix. There hollow spheres were mixed with an aluminum phosphate paste to form a sliarry, followed by molding to the required shape.
Fillers have also been used by Naik et al., in U.S. Pat. No. 5,064,727. There, abradable stationary seal walls, for jet turbine housings which seal opposing, rotating rotor blade tips, have a ceramic core containing from 30 vol. % to 98 vol. % solid ceramic filler, where the ceramic fills a honeycomb wall structure. This is then covered with erosion and corrosion resistant outer layer, which is made porous by uniformly dispersed, finely divided filler. This is apparently applied by a spraying technique. The pores can be filled with ceramic, metal oxide or carbide materials. Fillers mentioned include hollow ZrO2.8YO3 ceramic spheres and solid Al2O3,SiC,TiC and BN spheres.
Other abradable honeycomb structures for use in turbines are taught in U.S. Pat. No. 4,867,639 (Strangman). There, low melting fluorides, such as BaF2, are incorporated into a stabilized zirconia or alumina matrix which, in turn, is used to fill a honeycomb shroud lining made of, for example, a metal alloy. The filling becomes molten when the rotating blade tips rub the shroud, and upon resolidification, improve the smoothness of the abraded surface. Ainsworth et al., in U.S. Pat. No. 4,639,388, teaches another variation of reinforced ceramic layers, including a honeycomb matrix for use in a turbine as abradable seals.
In U.S. patent application Ser. No. 09/261,721 (RDM 97-017, Merrill et al., filed on Mar. 3, 1999), a honeycomb structure having open cells was filled, and optionally overlayed, with a material containing hollow ceramic particles embedded in an interconnected ceramic matrix, to provide a composite thermal barrier coating system having superior erosion resistance and abrasion properties for use on combustion turbine components. The hollow particles were preferably spherical and made of zirconia, alumina, mullite, ceria, YAG or the like, having an average particle size of about 200 micrometers (0.2 mm) to 1500 micrometers (1.5 mm). The steady state erosion rate, grams lost/kg erosive impacting media, of this filler was 3.2 g/kg vs. 4.6 to 8.6 g/kg for conventional TBCs. Here, the ceramic matrix comprised an interconnected skeleton binding the hollow spheres together where the hollow spheres were bonded by a network of aluminum phosphate bridging bonds. The matrix was applied as a slurry mixture of hollow particles, filler and binder, which was heated within the honeycomb structure to form a packed interconnecting ceramic matrix with embedded hollow particles. The flowable ceramic filler material was preferably packed into the open cells of the honeycomb using a combination of agitation and manually assisted packing using pushrods or tamping brushes to force pack the hollow spheres into the honeycomb cells ensuring complete filling. Alternate packing methods such as vacuum infiltration, metered doctor blading and similar high volume production methods were also mentioned.
In U.S. patent application Ser. No. 09/267,237 (99E9112US, Merrill et al., filed on Dec. 20, 1999), a material system useful as an erosion resistant layer for turbine applications was described. There, closely packed hollow, geometric shapes, such as hollow spheres were mixed with binder and other particles and bonded together with a matrix material to provide abradable, porous, thermally stable seals, and the like.
However, none of these coatings or seal structures have optimized abradability with erosion resistance and insulating capability, minimized shrinkability and thermal mismatch, provided constrained stabilized porosity and adequate flexibility, and optimized thermal stability at over 1600° C.; all of which characteristics are required of the next generation high temperature turbine TBCs, blade tip coatings and seals. What is needed is a method of making high temperature turbine coatings and composites that fill these requirements.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.
Therefore, it is one of the main objects of this invention to provide a method making a high temperature, erosion resistant coating and material which is bondable, non-shrinking, abradable, flexible, thermally stable up to at least 1600° C., and which has constrained stabilized porosity and insulating properties, as well as controlled thermal conductivity and thermal expansion properties.